1. Field of the Invention
The present invention relates to an electronic control unit and a manufacturing method of the same. The present invention is applicable to an electronic control unit used in an electric power-assisted steering system.
2. Description of Related Art
There is known an electric power-assisted steering system for assisting a driver in steering. In a typical electric power-assisted steering system, a motor is rotated only when a force for steering assistance is needed. Thus, compared to a hydraulic power-assisted steering system, a typical electric power-assisted steering system is fuel-saving and environmentally friendly because of no waste oil.
A typical electric power-assisted steering system requires a large current of, for example, about 100 A to drive a motor when a vehicle has a low speed and a large steering angle, e.g., when a vehicle is moved into a garage. Thus, a power MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) used in an ECU (electronic control unit) for drive control of the motor can instantaneously have a junction temperature between, for example, about 150 degrees C. and about 170 degrees C.
In recent years, an engine room of a vehicle or an engine room side of an instrument panel, in which the ECU is typically disposed, has become a small space because of an increase in space of a vehicle compartment and an increase in the number of other ECUs for controlling various parts of the vehicle. If the ECU is downsized in view of the above, the ECU may have a high-density circuit, which disadvantageously lowers a heat radiation performance.
According to JP-H6-3832A, a molybdenum sheet is disposed between a heat sink and a board having a nickel-plated silicon-carbide, so that the heat generated by an electronic component mounted to the board is conducted to the heat sink. However, when the board uses such a high-priced material, the manufacturing cost becomes disadvantageously large.